Along with the trend toward miniaturization and multi-functioning of various audio devices and communications devices, miniaturization of loudspeakers to be used in these devices is increasingly accelerated. Especially for loudspeakers to be used in mobile phones, not only miniaturization but also high-quality sound is required. Under such circumstances, emphasis of loudspeaker development is placed on rectangular loudspeakers rather than more general circular loudspeakers to enable more efficient use of limited space available in mobile phones where loudspeaker mounting space is limited.
FIG. 6 is a cross-sectional view showing structure of a conventional loudspeaker of this type. FIG. 7 is a perspective view showing structure of a yoke used in the loudspeaker. In FIG. 6 and FIG. 7, by providing four folding portions 11a on a previously blanked sheet of a magnetic metallic material and folding them up, yoke 11 is formed into a shape of a bottomed rectangular frame. With this, reduction in material loss and improvement in productivity of yoke 11 can be achieved. Also, by stacking and joining rectangular magnet 12 and rectangular top plate 13 inside yoke 11, a magnetic circuit having rectangular magnetic gap 14 is constructed.
The magnetic circuit and frame 15 in which yoke 11 is press fit are joined with an adhesive. Voice coil 16 is provided in a manner fitting in magnetic gap 14. Central portion of diaphragm 17 couples with voice coil 16 while its peripheral border couples with the peripheral border of frame 15.
In a conventional loudspeaker configured in this way, when an audio signal is input into voice coil 16 fit inside magnetic gap 14, voice coil 16 vertically vibrates due to a magnetic field generated by magnetic gap 14. This further causes vertical vibration of diaphragm 17 to which voice coil 16 is coupled thereby reproducing sound.
As a prior art technical literature related to the invention of this application, Unexamined Japanese Patent Publication 2003-37892, for example, is known.
However, as the above-mentioned conventional loudspeaker is fabricated by a method of preparing yoke 11 that configures a magnetic circuit by bending a sheet of a magnetic metallic material, a clearance is caused at each corner of yoke 11 formed into a rectangular configuration. Accordingly, magnetic flux leakage takes place from the clearances resulting in performance degradation such as reduction in the sound pressure due to reduction in magnetic flux density. There is also a fear of an adverse effect being caused by leakage magnetic flux on other electronic components densely mounted in the vicinity of the loudspeaker.